1. Field of the Invention
The present invention relates to a drive device for driving a single-cluster lamp composed of one LED or several LEDs to emit light.
2. Description of Related Art
A light emitting diodes (LED) is a light emitting device made of semiconductor material. It has two electrode terminals. If a voltage is applied across these two terminals and a very small current is provided, light will be emitted through recombination of electrons and holes therein. This is the light emission principle of the LED.
Different from common incandescent bulbs, the LED belongs to cold light emission devices, and has the advantages of low power consumption, long lifetime, no warm-up time required, and fast response speed. It has a small size, can endure shocks, and is suitable for mass production. Moreover, LEDs can be made into a very small or arrayed device to meet the requirements in application. Today, LEDs have been widely used on pointers and display devices of information, communication and consumer electronics products. They have become important components in everyday life of people.
LEDs are primarily used in traffic lights, car indication lights, brake lights, and so on. A full-color LED display makes use of a display screen composed of red, green, and blue (the three primary colors) LEDs, and is widely used as a stadium billboard, a street advertisement billboard, and so on. Along with popularization of mobile phones and other portable electronic products, LEDs have become indispensable light emitting devices. Because LEDs have the advantages of small size, fast on speed, and long lifetime, they will replace part of lighting in the future.
Because the application of LED is universal, various kinds of drive circuits and drive chips come into being. FIG. 1 is a voltage versus current characteristic curve of an ordinary diode. Except for some specific usages, a diode generally operates under forward bias. The relationship between the voltage and current of diode is expressed with an exponential curve. When the voltage across two terminals of a diode exceeds a certain voltage Vf (which is usually termed as the cut-in voltage or the threshold voltage, and is about 0.5-0.8V, depending on the diode material and doping concentration), the current of the diode increases abruptly. Before this cut-in voltage, the current in the diode is very small. The characteristics of an LED is the same as those of a diode. As its name implies, an LED will emit light when it is forward biased.
LEDs are usually made of GaAsP or GaP. With different materials, the emission wavelengths and thus colors will be different. For example, a GaAsP LED emits red light, while a GaP LED emits a light color between yellow and green. The luminance and conduction current of an LED is in a proportional relation, as shown in FIG. 2. Therefore, in order to keep a certain brightness in practical applications, the drive circuit is generally designed to be of constant current driving. This will also lengthen the use lifetime of an LED.